Local Oscillator Phase Synchronization for Beamforming and MIMO

ABSTRACT

An initial phase of each output signal generated by a plurality of radio frequency (RF) front-end circuits is determined by mixing an input signal with a mixing signal in a mixer of the corresponding RF front-end circuit. To that end, a time difference for each of the plurality of RF front-end circuits is determined by measuring a time difference between a reference signal (common to all of the RF front-end circuits) and the mixing signal of each RF front-end circuit. The initial phase for each output signal is then determined based on the measured time difference for the corresponding RF front-end circuit. Determining the initial phase in this manner accounts for any uncertainty of the phase when the RF front-end circuits are activated, enabling the phase of the corresponding antenna element to be accurately controlled.

BACKGROUND

Beamforming systems in wireless networks, e.g., beamformingtransmitters, beamforming receivers, etc., provide directional signalcontrol by combining signals transmitted or received by two or moreantenna elements of an antenna array such that signals at particularangles experience constructive interference, while other signalsexperience destructive interference. Such directional control providesimproved coverage and less interference in the wireless network. Forsimplicity, the following refers to beamforming transmitters. It will beappreciated, however, that the problems and solutions discussed hereinapply to any element utilizing beamforming or phase control, includingbeamforming receivers.

The directional control of beamforming transmitters may be achieved bycontrolling the phase and relative amplitude of the signal applied toeach antenna element. Thus, the performance of the beamformingtransmitter is inextricably tied to the accuracy of the phase control ofeach antenna element. Some systems implement local oscillatorbeamforming, which involves phase shifting the local oscillator (LO)signal for each antenna element to achieve the desired phase shift forthe corresponding antenna element. Such phase control of the LO signalrequires good control of the static phase differences between theseparate transmitters.

The static phase differences between the transmitters, however, can bedifficult to predict. For example, an integrated direct upconversionradio typically produces the upconversion mixing signal by dividing theLO signal with, e.g., a digital quadrature frequency divider. Such adivider may start up in any one of its possible internal states, whereeach state is associated with a different phase. This is a problem forbeamforming transmitters, particularly in time-division duplex (TDD)systems, where each transmitter is powered down between transmissionslots. Because each antenna element has its own transmitter, theresulting radiation pattern will constantly change between thetransmission slots because the initial phase for each antenna element isunknown every time the transmitter is powered back on. This same problemalso exists for TDD beamforming receivers, and transmitters and/orreceivers in Multiple Input, Multiple Output (MIMO) systems. Thus, thereremains a need for accounting for such start-up phase variations whencontrolling the phase of the antenna signals.

SUMMARY

The solution presented herein determines an initial phase of each outputsignal generated by a plurality of radio frequency (RF) front-endcircuits by mixing an input signal with a mixing signal in a mixer ofthe corresponding RF front-end circuit. To that end, the solutionpresented herein measures a time difference for each of the plurality ofRF front-end circuits by measuring a time difference between a referencesignal (common to all of the RF front-end circuits) and each mixingsignal. The initial phase for each output signal is then determinedbased on the measured time difference for the corresponding RF front-endcircuit. In so doing, the solution presented herein accounts for anyuncertainty of the phase when the RF front-end is activated, enablingthe phase of the corresponding antenna element to be accuratelycontrolled.

One exemplary embodiment provides a method of determining an initialphase of each output signal generated by a plurality of RF front-endcircuits, each of said RF front-end circuits comprising a mixerconfigured to mix an input signal with a mixing signal to generate theoutput signal for that RF front-end circuit. The method comprisesgenerating a reference signal common to the plurality of RF front-endcircuits. For each of the plurality of RF front-end circuits, the methodfurther comprises measuring a time difference between a start edge ofthe reference signal and a stop edge of the corresponding mixing signal.The start edge defines a beginning of a measurement period and each stopedge defines an end of the measurement period for the corresponding RFfront-end circuit. The method further comprises determining the initialphase of each output signal from the time difference measured for thecorresponding RF front-end circuit.

An exemplary wireless transceiver comprises a plurality of RF front-endcircuits, a reference circuit, and a plurality of time measurementcircuits, one for each RF front-end circuit. Each of the RF front-endcircuits comprises a mixer configured to mix an input signal with amixing signal to generate an output signal for that RF front-endcircuit. The reference circuit is configured to generate a referencesignal common to the plurality of RF front-end circuits. Each timemeasurement circuit is operatively connected to an input of thecorresponding mixer and the reference circuit. Further, each timemeasurement circuit is configured to measure a time difference between astart edge of the reference signal and a stop edge of the mixing signalof the corresponding RF front-end circuit. The start edge defines abeginning of a measurement period, and each stop edge defines an end ofthe measurement period for the corresponding RF front-end circuit. Thewireless transceiver is configured to determine the initial phase ofeach output signal from the time difference measured for thecorresponding RF front-end circuit.

In another exemplary embodiment, a computer program product stored in anon-transitory computer readable medium controls a phase of each of aplurality of radio frequency (RF) front-end circuits of a wirelesstransceiver. The computer program product comprises softwareinstructions which, when run on the wireless transceiver, causes thewireless transceiver to generate an output signal for each RF front-endcircuit by mixing an input signal with a mixing signal, and generate areference signal common to the plurality of RF front-end circuits. Thesoftware instructions further cause the wireless transceiver to measurea time difference between a start edge of the reference signal and astop edge of the mixing signal of the corresponding RF front-endcircuit. The start edge defines a beginning of a measurement period andwherein each stop edge defines an end of the measurement period for thecorresponding RF front-end circuit. The software instructions furthercause the wireless transceiver to determine an initial phase of eachoutput signal from the time difference measured for the corresponding RFfront-end circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary block diagram for beamforming system.

FIG. 2 shows a block diagram for a beamforming transmitter according toone exemplary embodiment.

FIG. 3 shows an exemplary block diagram for a transmission RF front-endfor the beamforming system of FIG. 1.

FIG. 4 shows a phase determination method according to one exemplaryembodiment.

FIG. 5 shows an exemplary signaling diagram for determining the initialphase according to one exemplary embodiment.

FIG. 6 shows an exemplary signaling diagram for determining the initialphase according to another exemplary embodiment.

FIG. 7A shows one exemplary circuit implementation for the TDC.

FIG. 7B shows an exemplary signaling diagram for the TDC of FIG. 7A.

FIG. 8 shows a block diagram for a beamforming transmitter according toanother exemplary embodiment.

FIG. 9 shows an exemplary signaling diagram for determining the initialphase according to another exemplary embodiment.

FIG. 10 shows an exemplary block diagram for a reception RF front-endfor the beamforming system of FIG. 1.

FIG. 11 shows a block diagram for a beamforming receiver according toone exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a beamforming system 10 comprising an antenna array 20 withM antenna elements 22, where the m^(th) antenna element 22 is coupled tothe m^(th) radio frequency (RF) front end 30. Each RF front end 30comprises a phase-locked loop (PLL) 32, as shown in FIG. 2. One way tocontrol the direction of the beam for the antenna element 22 is to havethe PLL 32 control the phase of the PLL output signal responsive to aninput control signal input to the PLL 32 relative to the referenceclock. In some embodiments, the PLL control signal indicates the desiredphase shift of the PLL output signal, where the desired phase shift wasdetermined by a controller (not shown) external to the PLL 32, e.g., anarray controller, based on a location of the antenna element 22 in theantenna array 20, a desired beam direction, and/or a desired beam shape.In other embodiments, the PLL control signal indicates the location ofthe antenna element 22 in the antenna array 20, the desired beamdirection, and/or the desired beam shape, and the PLL 32 determines orotherwise selects the desired phase shift based on the informationprovided by the PLL control signal. The phase of the transmitter, andthus the direction of the beam, may also or additionally be controlledby rotating the phase of the digital or analog baseband signal appliedto the corresponding RF front-end 30. For example, the phase of thedigital baseband signal may be rotated by a desired amount. Afterrotation, the digital baseband signal may be converted to an analogsignal and applied to the corresponding RF front-end 30 for upconversionand transmission.

It will be appreciated that FIG. 1 shows a simplified block diagram ofthe exemplary beamforming system 10. Other components not pertinent tothe discussion have been excluded from the drawings for simplicity.

The following will first describe the embodiments disclosed herein interms of a beamforming transmitter 100, such as shown in FIG. 3. It willbe appreciated, however, that the embodiments disclosed herein apply toother beamforming elements, e.g., beamforming receivers (FIGS. 10 and11), and/or any similar system requiring knowledge of an initial phasefor phase control.

FIG. 2 shows a block diagram of an exemplary RF front-end 30, e.g., them^(th) RF front-end of the beamforming system 10 shown in FIG. 1. EachRF front-end 30 comprises a phase-locked loop (PLL) 32, a divider 34operatively coupled to an output of the PLL 32, an upconversion mixer 36operatively coupled to an output of the divider 34, and an amplifier 38operatively coupled to an output of the mixer 36. PLL 32 outputs asignal at a desired PLL frequency f_(LO) responsive to a referencesignal. Divider 34 divides the frequency f_(LO) of the PLL output signalby N to achieve a mixing signal S_(mix) at a desired radio frequency,where N may be a fraction or an integer. Upconversion mixer 36upconverts an input signal S_(I) responsive to the frequency of themixing signal S_(mix). An amplifier 38, e.g., a power amplifier,amplifies the upconverted signal to generate the output signalS_(out)(m), e.g., a transmission signal for transmission by thecorresponding antenna element 22.

As shown in FIG. 3, transmitter 100 also includes a reference circuit 40and plurality of time measurement circuits 50, e.g., a Time-to-DigitalConverter (TDC), one for each RF front-end circuit 30. The referencecircuit 40 is configured to generate a reference clock signal S_(R)common to the plurality of RF front-end circuits 30. Reference circuit40 may comprise any known clock circuit capable of generating areference clock signal at the desired frequency. Further, while notrequired, the reference clock signal S_(R) output by reference circuit40 may also be used as the phase-locked loop reference signal providedto the phase-locked loop 32 in each RF front-end circuit 30.

Each TDC 50 is operatively connected to the reference clock circuit 40to receive the common reference clock signal S_(R), and to thecorresponding RF front-end circuit 30 to receive the mixing signalS_(mix) generated by that RF front-end circuit 30. Each TDC 50 measuresa time difference related to the current phase of the correspondingmixing signal S_(mix), and determines the initial phase for the outputof the corresponding RF front-end circuit 30 based on the measured timedifference. By using a common reference signal for all time differencemeasurements/initial phase determinations, the solution presented hereinmakes sure that the time difference measurements executed by each TDC 50occur simultaneously, e.g., with the next edge of the reference clocksignal. As a result, the correct initial phase can be determined foreach output signal. The TDC 50 may output the time difference, or maycompute the initial phase from the time difference, e.g., according toEquation (1), and output the computed initial phase.

More particularly, FIG. 4 shows an exemplary method 200 for determiningthe initial phase of each output signal S_(out) generated by each of theplurality of RF front-end circuits 30. After reference circuit 40generates the reference signal S_(R) (block 210), the TDC 50 measuresthe time difference Δt(m) for the m^(th) RF front-end circuit 30 bymeasuring a difference between a start edge t_(edge)(S_(R)) of thereference clock signal S_(R) and a stop edge t_(edge)(S_(mix)(m)) of themixing signal S_(mix) (block 220) for the m^(th) RF front-end circuit30. The transmitter 100 determines the initial phase of the outputsignal, in this case transmission signal S_(out)(m), output by each RFfront-end circuit 30 as a function of the corresponding measured timedifference (block 230).

FIGS. 5 and 6 show signaling diagrams illustrating the time measurementprocess of FIG. 4 according to various embodiments. The TDC 50 triggerson an edge of the reference clock signal S_(R), e.g., a rising edge, andmeasures how long after that rising edge the mixing signal S_(mix) has arising edge. In FIG. 5, the TDC stops the measurement window at the nextrising edge of the mixing signal S_(mix) after detecting the rising edgeof the reference clock signal S_(R). In FIG. 6, the TDC 50 stops themeasurement window at a rising edge of the mixing signal S_(mix) severalclock cycles after detecting the rising edge of the reference clocksignal S_(R). The measurement of the time difference over multiple LOclock cycles, such as shown in FIG. 6, may be used to avoidmetastability in the triggering logic without adding complexity to thesolution or impacting the accuracy of the measurement. In either case,the initial phase Δφ_(i)(m) for the m^(th) output signal S_(T)(m) may becalculated according to:

$\begin{matrix}{{{\Delta\varphi}_{i}(m)} = {{2\pi \frac{\Delta \; {t(m)}}{f_{LO}(m)}} = {2\pi \frac{{n_{TDC}(m)}{\Delta step}}{f_{LO}(m)}}}} & (1)\end{matrix}$

where n_(TDC)(m) represents the number of TDC quantization stepsmeasured by the m^(th) TDC 50 between the start edge (e.g., thereference clock rising edge t_(edge)(S_(R))) and the stop edge (e.g.,the mixing signal rising edge t_(edge)(S_(mix))), and Δstep representsTDC quantization step size in seconds. It will be appreciated that theembodiments disclosed herein do not require the TDC 50 to use the risingedges of the mixing and reference clock signals; any signaling edge,e.g., a falling edge, may alternatively be used.

FIG. 7A shows one exemplary circuit/logic implementation for the TDC 50,while FIG. 7B shows one exemplary signaling diagram corresponding to theTDC 50 of FIG. 7A. As noted herein, the arm signal is not required forall embodiments. The configuration of AND, XOR, and timing gates shownin FIG. 7A perform the corresponding logic functions on the inputsignals (S_(R), S_(mix), and optionally, the arm signal) to trigger thebeginning and end of the measurement window, and thus to enable themeasurement of the duration of the measurement window. Because each TDC50 has the same input reference clock signal S_(R), and the same armsignal (when used), the determination of the duration of the measurementwindow enables the initial phase to be simultaneously determined foreach output signal.

The transmitter 100 uses the determined initial phase Δφ_(i)(m) tofacilitate accurate phase control of the corresponding antenna element22. To that end, the transmitter 100 may also include a phase controlcircuit 60 for each RF front-end circuit 30, as shown in FIG. 3. Thephase control circuit 60 may receive the determined initial phaseΔφ_(i)(m) from the corresponding TDC 50. Alternatively, the phasecontrol circuit 60 may receive the measured time difference Δt(m) fromthe corresponding TDC 50, and then compute the initial phase Δφ_(i)(m)from the received measured time difference. In any event, the phasecontrol circuit 60 uses the initial phase Δφ_(i)(m) to initialize or“calibrate” the phase of the corresponding m^(th) RF front-end circuit30 so that the m^(th) RF front-end circuit 30 executes any phase controlactivities from a known initial phase. The phase control circuits 60 maythen adjust, e.g., rotate, the initial phase determined for that outputsignal to achieve the desired phase value for that output signal. Forexample, the phase control circuit 60 may use the initial phase toadjust the input control signal used to control the phase of the PLLoutput signal. Alternatively, the phase control circuit 60 may use theinitial phase to control/adjust the phase of the baseband signal S_(I).Because the phase control circuit 60 effectively starts the phaseadjustment from the known initial phase value, the resulting phasecontrol is accurate and reliably achieves the desired performance, e.g.,beamforming.

FIG. 3 involves a transmitter with all of the RF front-end circuits 30on a single integrated circuit. The solution presented herein, however,may also be used when different RF front-end circuits 30 are disposed ondifferent integrated circuits, such as shown in FIG. 8. In such cases,transmitter 100 will further include a synchronization circuit 70 foreach integrated circuit. Each synchronization circuit 70 may beimplemented, e.g., using a counter running with a clock common to all ofthe integrated circuits. The synchronization circuits 70 synchronize thetiming measurements occurring on different integrated circuits. Tosynchronize the timing measurements, each synchronization circuit 70provides an arm signal, such as shown in FIG. 9, to the TDCs 50 on thatintegrated circuit. Further, the synchronization circuits 70 are alsosynchronized across the integrated circuits to makes sure that the armsignals provided to each TDC 50 in the transmitter 100 are synchronized.As shown in FIG. 9, once the arm signal is activated, e.g., goes high,the TDC 50 triggers on an edge of the reference clock signal S_(R),e.g., a rising edge, and measures how long after that rising edge themixing signal S_(mix) has a rising edge to determine Δt(m). The initialphase Δφ_(i)(m) for the m^(th) output signal S_(out)(m) may becalculated according to Equation (1). While the above-described examplesonly use the synchronization circuit 70 and corresponding “arm” signalswhen different circuits of the transmitter 100 are disposed on differentintegrated circuits, the use of the synchronization circuit 70 is not solimited. It will be appreciated that the transmitter 100 may usesynchronization circuit(s) 70 and the corresponding arm signal(s) forany transmitter configuration, including when all of the RF front-endcircuits 30 are disposed on a single integrated circuit.

As noted above, the solution presented herein also applies tobeamforming receivers. In this case, the phase control circuit controlshow the received signals are combined to achieve the desired beamformingbenefits.

FIG. 10 shows a block diagram of the m^(th) antenna element 22, m^(th)receiving RF front-end 30, and the corresponding m^(th) TDC 50. In thiscase, the antenna element 22 provides a received signal to an amplifier39, which generates an amplified input signal S_(I) for the mixer 36 ofthat RF front-end circuit 30. Like with the transmitter embodiment, areference circuit 40 (FIG. 11) generates a reference clock signal S_(R)common to the plurality of RF front-end circuits 30. Further, while notrequired, the reference clock signal S_(R) output by reference circuit40 may also be used as the phase-locked loop reference signal providedto the phase-locked loop 32 in each RF front-end circuit 30.

Each TDC 50 is operatively connected to the reference clock circuit 40to receive the common reference clock signal S_(R), and to thecorresponding RF front-end circuit 30 to receive the mixing signalS_(mix) generated by that RF front-end circuit 30. Each TDC 50 measuresa time difference related to the current phase of the correspondingmixing signal S_(mix), and determines the initial phase for the outputof the corresponding RF front-end circuit 30 based on the measured timedifference. For reception beamforming, this involves the TDC 50measuring the time difference Δt(m) for the m^(th) RF front-end circuit30 by measuring a difference between a start edge t_(edge)(S_(R)) of thereference clock signal S_(R) and a stop edge t_(edge)(S_(mix)(m)) of themixing signal S_(mix) for the m^(th) RF front-end circuit 30. Thetransmitter 100 determines the initial phase of the output signal, inthis case reception output signal S_(out)(m) output by each RF front-endcircuit 30, as a function of the corresponding measured time difference.

FIG. 11 shows another block diagram of a beamforming receiver thatincludes synchronization circuit 70. Synchronization circuit 70 providesan arm signal to the corresponding TDCs 50. Further, if there aremultiple synchronization circuits 70, each of the synchronizationcircuits are also synchronized across the integrated circuits to makessure that the arm signals provided to each TDC 50 in the transmitter 100are synchronized. Once the arm signal is activated, e.g., goes high, theTDC 50 triggers on an edge of the reference clock signal S_(R), e.g., arising edge, and measures how long after that rising edge the mixingsignal S_(mix) has a rising edge to determine Δt(m). The initial phaseΔφ_(i)(m) for the m^(th) output signal S_(T)(m) may be calculatedaccording to Equation (1).

The solution presented herein provides an efficient and cost-effectiveway to determine the initial phase for scenarios where the phase causedby RF circuit components, e.g., a frequency divider, changesperiodically, e.g., each time the circuit is powered on. Bysimultaneously determining the initial phase of the output signal ofmultiple RF circuits, the solution presented herein enables accuratephase control.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

1. A method of determining an initial phase of each output signalgenerated by a plurality of radio frequency (RF) front-end circuits of abeamforming or multiple input, multiple output (MIMO) system, each ofsaid RF front-end circuits comprising a mixer tuned to a commonfrequency and configured to mix an input signal with a mixing signal togenerate the output signal for that RF front-end circuit, the methodcomprising: generating a reference signal common to the plurality of RFfront-end circuits; for each of the plurality of RF front-end circuits,measuring a time difference between a start edge of the reference signaland a stop edge of the corresponding mixing signal, wherein the startedge defines a beginning of a measurement period and wherein each stopedge defines an end of the measurement period for the corresponding RFfront-end circuit; determining the initial phase of each output signalfrom the time difference measured for the corresponding RF front-endcircuit; and using the initial phase to set a final phase of thecorresponding output signal to a desired value; wherein each of theplurality of RF front-end circuits comprises: transmission RF front-endcircuits, where each output signal comprises a transmission signalapplied to an antenna element of an antenna array; or receptionfront-end circuits, where each input signal comprises a reception signalprovided by an antenna element of an antenna array.
 2. The method ofclaim 1 further comprising rotating the initial phase of at least one ofthe output signals to set a final phase of that output signal to adesired phase value determined for that output signal.
 3. The method ofclaim 1 further comprising deriving the mixing signal from a localoscillator signal and a frequency divider.
 4. The method of claim 3wherein the reference signal comprises a phase-locked loop referencesignal applied to a phase-locked loop in each RF front-end circuit, andwherein the phase-locked loop generates the local oscillator signalbased on the reference signal.
 5. The method of claim 1 wherein thestart and stop edges both comprise rising edges, and wherein the stopedge comprises the next rising edge of the corresponding mixing signaloccurring immediately after the start edge.
 6. The method of claim 1wherein the measurement period spans multiple periods of the mixingsignal.
 7. The method of claim 1 further comprising arming the RFfront-end circuits to synchronize the measuring of the time differencesfor each of the plurality of RF front-end circuits.
 8. (canceled) 9.(canceled)
 10. A wireless circuit of a beamforming or multiple input,multiple output (MIMO) system, the wireless circuit comprising: aplurality of radio frequency (RF) front-end circuits of the beamformingor the multiple input, multiple output (MIMO) system, each of said RFfront-end circuits comprising a mixer tuned to a common frequency andconfigured to mix an input signal with a mixing signal to generate anoutput signal for that RF front-end circuit; a reference circuitconfigured to generate a reference signal common to the plurality of RFfront-end circuits; and a plurality of time measurement circuits, onefor each RF front-end circuit, each time measurement circuit operativelyconnected to an input of the corresponding mixer and the referencecircuit and configured to measure a time difference between a start edgeof the reference signal and a stop edge of the mixing signal of thecorresponding RF front-end circuit, wherein the start edge defines abeginning of a measurement period and wherein each stop edge defines anend of the measurement period for the corresponding RF front-endcircuit; wherein the wireless circuit is configured to determine theinitial phase of each output signal from the time difference measuredfor the corresponding RF front-end circuit, and to use the initial phaseto set a final phase of the corresponding output signal to a desiredvalue; wherein each of the plurality of RF front-end circuits comprises:transmission RF front-end circuits, where each output signal comprises atransmission signal applied to an antenna element of an antenna array;or reception front-end circuits, where each input signal comprises areception signal provided by an antenna element of an antenna array. 11.The wireless circuit of claim 10 further comprising a phase controlcircuit configured to rotate the initial phase of at least one of theoutput signals to set a final phase of that output signal to a desiredphase value determined for that output signal.
 12. The wireless circuitof claim 10 wherein each RF front-end circuit further comprises: a localoscillator implemented as a phase-locked loop; and a frequency divideroperatively coupled to an output of the phase-locked loop; wherein themixing signal is derived from an output of the frequency divider. 13.The wireless circuit of claim 12 wherein the reference signal furthercomprises a phase-locked loop reference signal applied to thephase-locked loop.
 14. The wireless circuit of claim 10 wherein thestart and stop edges both comprise rising edges, and wherein the stopedge comprises the next rising edge of the corresponding mixing signaloccurring immediately after the start edge.
 15. The wireless circuit ofclaim 10 wherein the measurement period spans multiple periods of themixing signal.
 16. The wireless circuit of claim 10 further comprisingat least one synchronization circuit configured to synchronize theoperations of the time measurement circuits to synchronize the measuringof the time differences for each of the plurality of RF front-endcircuits by applying a common arming signal to each of the timemeasurement circuits.
 17. The wireless circuit of claim 10 wherein eachtime measurement circuit determines the initial phase of thecorresponding output signal based on the time difference measured forthe corresponding RF front-end circuit.
 18. The wireless circuit ofclaim 10 further comprising a plurality of phase control circuits, onefor each RF front-end circuit, each phase control circuit configured todetermine the initial phase of the corresponding output signal based onthe time difference measured for the corresponding RF front-end circuit.19. (canceled)
 20. (canceled)
 21. A computer program product stored in anon-transitory computer readable medium for controlling a phase of eachof a plurality of radio frequency (RF) front-end circuits of a wirelesscircuit of a beamforming or multiple input, multiple output (MIMO)system, the computer program product comprising software instructionswhich, when run on the wireless circuit, causes the wireless circuit to:generate an output signal for each RF front-end circuit by mixing aninput signal with a mixing signal tuned to a common frequency; andgenerate a reference signal common to the plurality of RF front-endcircuits; measure a time difference between a start edge of thereference signal and a stop edge of the mixing signal of thecorresponding RF front-end circuit, wherein the start edge defines abeginning of a measurement period and wherein each stop edge defines anend of the measurement period for the corresponding RF front-endcircuit; determine an initial phase of each output signal from the timedifference measured for the corresponding RF front-end circuit; and usethe initial phase to set a final phase of the corresponding outputsignal to a desired value; wherein each of the plurality of RF front-endcircuits comprises: transmission RF front-end circuits, where eachoutput signal comprises a transmission signal applied to an antennaelement of an antenna array; or reception front-end circuits, where eachinput signal comprises a reception signal provided by an antenna elementof an antenna array.
 22. A wireless transceiver of a beamforming ormultiple input, multiple output (MIMO) system comprising: a plurality ofradio frequency (RF) front-end circuits of the beamforming or themultiple input, multiple output (MIMO) system, each of said RF front-endcircuits comprising a mixer tuned to a common frequency and configuredto mix an input signal with a mixing signal to generate an output signalfor that RF front-end circuit; a reference circuit configured togenerate a reference signal common to the plurality of RF front-endcircuits; and a plurality of time measurement circuits, one for each RFfront-end circuit, each time measurement circuit operatively connectedto an input of the corresponding mixer and the reference circuit andconfigured to measure a time difference between a start edge of thereference signal and a stop edge of the mixing signal of thecorresponding RF front-end circuit, wherein the start edge defines abeginning of a measurement period and wherein each stop edge defines anend of the measurement period for the corresponding RF front-endcircuit; wherein the wireless transceiver is configured to determine theinitial phase of each output signal from the time difference measuredfor the corresponding RF front-end circuit, and to use the initial phaseto set a final phase of the corresponding output signal to a desiredvalue; wherein each of the plurality of RF front-end circuits comprises:transmission RF front-end circuits, where each output signal comprises atransmission signal applied to an antenna element of an antenna array;or reception front-end circuits, where each input signal comprises areception signal provided by an antenna element of an antenna array.